Builder's Line: How to Keep Brickwork Straight (and the One Mistake That Bows Every Beginner's Wall)
The UK guide to builder's line. Line pins vs corner blocks, the 4-metre sag rule, tingle plates, and what to buy from [Unknown price: builders-line-budget] upwards.
A bricklayer builds a 6-metre garden wall freehand, checking each brick with a spirit level. The wall looks fine from a metre away. Stand at one end and sight along it, and the face ripples like a ribbon. Bricks creep outward by 2-3mm per course, and over ten courses that's a visible bow. The wall passes no quality inspection and needs rebuilding. A £5 piece of string would have prevented the entire problem.
What it is and when you need one
A builder's line is a length of strong, thin string stretched taut between two fixed points to act as a straight reference for laying bricks or blocks. You anchor each end to the corners of the wall you're building, pull the string tight, and lay each brick so its face runs parallel to the line. When you finish a course, you raise the line by one brick height and repeat.
The string itself does nothing clever. It's just nylon or polyethylene cord, usually bright orange or white so you can see it against mortar. The skill is in how you anchor it and how you read it. The anchoring is done with either line pins (steel spikes pushed into mortar joints) or corner blocks (L-shaped plastic pieces that clip over the end brick). More on those in a moment.
A spirit level tells you whether individual bricks are level and plumb. A builder's line tells you whether the whole wall face is straight along its length. They do different jobs, and you need both. The level checks the corners. The line checks everything in between.
You'll need one for any bricklaying or blockwork, which in a typical extension means the entire superstructure: external walls, internal walls, garden walls, retaining walls. But a builder's line is also useful for setting out foundations (stretching it between profile boards to mark trench positions) and for checking alignment during snagging.
Types and variants
The string itself comes in three materials. The anchoring system comes in two main types. And there's one accessory that most guides never mention but that you'll need for any wall longer than about 4 metres.
The string
| Material | Price (100m) | Stretch | Durability | Best for |
|---|---|---|---|---|
| Polyethylene | £3-5 | Moderate | Adequate for a single project | Budget choice for a one-off job |
| Braided nylon | £5-10 | Low | Excellent - doesn't fray when cut | Repeated use across a full build |
| Cotton | £2-4 | High when wet | Poor - absorbs water, rots | Not recommended |
Braided nylon is the right choice. It stays taut in all weather, doesn't stretch when damp, and won't fray at the cut end. The Toolstation Minotaur braided nylon 100m roll costs £4.98 and the Screwfix Magnusson equivalent is £4.97. Either is more line than you'll use on an entire extension project.
Cotton line is what your grandad used. It absorbs moisture, stretches when wet, and goes slack on a rainy site. It's cheap but not worth the trouble. Polyethylene is fine for a weekend garden wall project, but braided nylon costs barely more and performs noticeably better.
Line pins vs corner blocks
These are the two methods for anchoring the string to your wall. They're alternatives, not complementary tools. Pick one system and use it.
Line pins are hardened steel spikes, about 160mm long, with a flat blade at one end and a point at the other. You push or tap the pointed end into a vertical mortar joint (the wet mortar between bricks), wind the string around the pin, and pull it taut to the matching pin at the opposite end of the wall. The flat blade holds the pin firm against the wall face.
Corner blocks (also called line blocks) are L-shaped plastic pieces that hook over the end brick at each corner. The string feeds through a narrow groove in the block. They're faster to reposition between courses because you just slide them up one brick height instead of pulling out a pin and re-driving it.
Which should you choose? For a homeowner doing their first brickwork, corner blocks are easier to use. They don't require you to judge mortar firmness or risk disturbing freshly laid bricks when you push a pin in. Line pins are more versatile because they work anywhere along a wall (not just at corners), which matters when you're building a long run and need a mid-wall anchor. Most professionals use pins from habit.
Tingle plates: the accessory nobody mentions
On any wall run longer than about 4 metres, the string sags under its own weight. Even pulled very tight, a 6-metre span of nylon line will dip noticeably in the middle. The accepted trade figure is about 25mm (one inch) of sag over a 30-metre run, but you'll notice it well before that. At 4 metres, the sag is enough to throw off your brickwork if you're working to the middle of the line.
The fix is a tingle plate (sometimes just called a tingle). It's a thin piece of metal or plastic with two vertical slots. You slide it under the line at the midpoint, rest it on a brick that's already been laid and checked for level, and the plate supports the line from below. The string stays straight across the full span.
You can buy tingle plates or improvise one on site: fold a piece of cement bag paper over the line and weigh it down with a brick on the course below. Professionals do this routinely.
The 4-metre rule is a practical limit, not a precise measurement. On a calm day with good nylon line pulled very tight, you can push to 5 metres without a tingle. In wind, or with cheaper polyethylene line, 3 metres is more realistic. When in doubt, use a tingle. It takes ten seconds to set up and costs nothing if you improvise.
How to use it properly
This is the section that separates a guide worth reading from a product listing. The technique matters far more than the tool.
Step 1: Build corner leads first
You never stretch a line and start laying bricks from one end to the other. The correct method is to build the corners (called leads) first, checking each corner brick for level and plumb with a spirit level. Build each corner up by 4-5 courses, carefully gauging the height so each course matches standard brick gauge (75mm per course for standard bricks, which is one 65mm brick plus a 10mm mortar joint).
Once your corners are plumb, level, and at the correct gauge, you stretch the line between them and fill in the courses between. The corners are your reference. The line transfers their accuracy across the whole wall.
Step 2: Set up the line
If you're using line pins, push the point of one pin into a perpend joint (the vertical mortar joint between bricks) at one corner, at the top edge of the course you're about to lay. Wind the string around the pin two or three times and pull it to the opposite corner. Push the second pin in at the same course height, pull the string taut, and wind it to secure.
If you're using corner blocks, tie a knot in one end of the string and feed it through the groove in the block until the knot catches. Hook the block over the end brick at your corner, with the groove sitting at the top edge of the course you're about to lay. Pull the string to the opposite corner, thread through the second block, and hook it over.
The string must sit at the top outside edge (called the arris) of the course being laid. Not above it, not below it, not inboard. Right at the top front corner of where the next row of bricks will go.
Step 3: Lay to the line (and the critical gap)
Here's the single most important rule, and the one that trips up every beginner.
Your bricks must never touch the string. Keep a gap of about 1-2mm between the front face of each brick and the line. A trowel blade's thickness is about right. If even one brick pushes the line outward, every subsequent brick laid to that displaced line will also be pushed out. Multiply this across a full course and you get a bowed wall. This is the most common cause of wavy wall faces in amateur brickwork.
Lay each brick so its front face is close to the line but not touching. You're using the line as a visual reference, not a physical stop. Sight along the line from one end: the face of every brick in the course should appear to sit at the same distance behind the string, forming a straight plane.
Step 4: Raise and repeat
When you finish a course, move the line up by one brick height. With line pins, pull them out, move them up one course, and re-drive. With corner blocks, slide them up the corner brick. Check that the string sits at the arris of the next course to be laid. Lay the next course.
Don't check every single brick with a spirit level. Check the corners (they must stay plumb) and run the level across every 3-4 courses to confirm gauge. The line handles straightness. The level handles level and plumb. Experienced bricklayers check level on the third course and the last course, and trust the line for everything else in between.
Beginners spend too long fussing over each brick, tapping and adjusting repeatedly. Lay the brick, check it's close to the line and roughly level, and move on. Mortar is forgiving at the individual brick level. Straightness across the whole course matters more than perfection on any single brick.
What to buy
A complete builder's line setup costs under £20. You need three things: the string, something to anchor it with, and (for walls over 4 metres) tingle plates.
Best value starter kit: Footprint 3-piece set. This gives you 100m of braided nylon line, a pair of line blocks, and a pair of line pins for around £10. Footprint is a Sheffield manufacturer that's been making bricklaying tools since the Victorian era. Their line blocks have a viewing window for gauge markings and narrow channels that grip the string firmly. Available at On-Site Tools and specialist merchants.
If buying separately from Screwfix or Toolstation:
- Magnusson or Minotaur 100m braided nylon line: £5 – £5
- Footprint line blocks (pair): £6.49
- Footprint line pins 160mm (pair): £8.49
- Total for all three: around £20
You don't need both line blocks and line pins on day one. Pick one anchoring method. If you're a complete beginner, start with corner blocks.
Budget floor: Silverline 18m line with corner blocks. Available from Sealants and Tools Direct for about £4.9. This is a polyethylene line (not braided nylon) with basic plastic corner blocks. Fine for a single garden wall project. Not robust enough for a full extension build where you'll be using the line across weeks of brickwork.
Premium pick: Crompton corner blocks with integrated line pin holder. At £18.5 for a pair, these are patent-protected magnetic corner blocks designed by a working bricklayer with over 20 years' experience. The string wraps around the block and the line pin is held magnetically. A clever design, but unless you're laying bricks professionally, the standard Footprint blocks do the same job for a third of the price.
Tingle plates: £2 – £5 for a set from Amazon or Etsy. Or improvise with a folded cement bag and a spare brick. Don't buy them separately if you're on a tight budget.
Alternatives
A laser level projects a visible horizontal or vertical line across a surface. Useful for establishing a datum (the initial reference height for your first course of bricks) and for checking that your finished wall is level over long distances. But lasers are slower to adjust between courses than simply raising a string, and the line disappears the moment you switch it off. String for active bricklaying, laser for setup and checking.
A chalk line snaps a straight mark onto a surface. Useful for marking out wall positions on a slab or foundation, but the mark is on the surface below, not at working height along the wall face. A chalk line and a builder's line do related but different jobs. You'll likely use both during a build.
Timber profiles are an alternative to line pins and corner blocks for DIY blockwork. Screw or clamp a straight piece of 100x50mm timber to each corner of your wall, check it's plumb, and mark gauge increments up its face (225mm for block courses). Attach the string at each mark with a screw or nail. This is slower to set up than corner blocks but gives you a permanent gauge reference at each corner. Several DIY bricklayers on forum threads recommend this approach for a first project.
Where you'll need this
A builder's line appears at multiple stages of any extension or renovation project:
- Walls and blockwork - the primary use, guiding every course of brickwork and blockwork in the superstructure
- Foundations and footings - stretched between profile boards to set out trench positions and building lines
- First fix electrics - marking consistent socket and switch positions along a wall run
- First fix plumbing - marking pipe run positions for consistent alignment
- Kitchen installation - checking wall straightness before fitting units, aligning worktop runs
- Snagging checklist - stretching a line along finished walls to check for bows and deviations
Safety
Builder's line isn't a dangerous tool, but two things catch people out.
A taut nylon line under tension can whip back if one end pulls free. It won't cause serious injury, but a line pin flying out of soft mortar and pinging across a site can hit someone in the eye. Wear safety glasses when working around freshly set line pins, especially if the mortar is still green.
Loose line on the ground is a trip hazard. Wrap excess string around the corner block or line pin, not piled on the floor. On a busy site with multiple trades, a loop of invisible nylon string across a walkway will put someone on the ground.